The correct schema is C. Rowkey: device_id Column data: date, data_point. This design is optimal because:

• Rowkey: device_id: Facilitates efficient retrieval of all data points for a specific device, aligning with the most common query requirement.
• Column data: date, data_point: Allows for efficient filtering by specific days or date ranges within a device's data, and stores the actual data record received every 15 minutes.

Other options are less suitable due to inefficiencies in retrieving data for specific devices or not aligning with the common query pattern.